The present invention relates to a focus error detector for detecting a signal representing a degree of a focus error of an objective lens with respect to an optical recording medium in an optical pickup for recording and/or reproducing information by projecting light onto the optical recording medium, and more particularly, to a focus error detector based on a critical angle method.
The critical angle method is well known as a method using a critical angle prism as a means for detecting a focus error in a recording and/or reproducing optical pickup. The optical structure of an optical pickup adopting a focus error detector based on the conventional critical angle method is shown in FIG. 1.
Referring to FIG. 1, the light radiated from a laser diode 1 used as a light source is collimated by means of a collimating lens 2 and then passes through a beam splitter 3 to then be incident to an objective lens 4. Thereafter, the light is focused onto an optical disk 5 by objective lens 4. After the reflected light from optical disk 5 is collimated again via objective lens 4, the light is reflected in beam splitter 3 to travel toward a critical angle prism 6 and be reflected on a critical plane 6a of critical angle prism 6 to ultimately reach a photo-detector 7. A differential amplifier 8 differentiates signals from split sections 7a and 7b of photo-detector 7. At this time, the signals of differential amplifier 8 are varied in their polarities and magnitudes according to the distance between objective lens 4 and optical disk 5 and the degree of focus errors of objective lens 4, which is shown in FIGS. 2A-2C and 3A-3C in detail.
FIG. 2A shows an on-focus state, that is, a state where optical disk 5 is on a focal plane of objective lens 4. At this time, the reflected light 9 which is collimated is totally reflected on the critical plane 6a of critical angle prism 6. Therefore, as shown in FIG. 3A, the reflected light 9 is equally received by both split sections 7a and 7b of photodetector 7. The signal value of differential amplifier 8 becomes zero.
However, if optical disk 5 deviates from the focal plane of objective lens 4, the reflected light 9 reflected from optical disk 5 is either converged or diverged, as shown in FIGS. 2B and 2C. At this time, the reflected light amount is reduced at a point A or B on the critical plane 6a of critical angle prism 6. Therefore, as shown in FIGS. 3B and 3C, the reflected light 9 is received at split section 7a or 7b of photo-detector 7, as a semicircular pattern. The signal of differential amplifier 8 is output as a negative or positive value.
In the meantime, for example, if the optical axis of the reflected light is tilted or shifted horizontally due to the optical disk vibration or changes thereto over time, the reflection efficiency of critical angle prism for the reflected light is varied. Therefore, in this case, even if optical disk is positioned on the focal plane of objective lens, the light amount received by two split sections of photo-detector becomes different. Finally, the signal value of differential amplifier becomes a value other than zero. That is, since the focus error detector based on the conventional critical angle method regards the shift or tilt of the optical axis of reflected light as an error, the optical pickup experiences frequent malfunction.